Obesity in humans remains a significant health problem in the United States. Research with humans and with animal models has established that obesity is multifaceted, involving altered energy expenditure and altered energy intake. Recent work has also shown that genetic background contributes to the development of obesity in humans and illustrates the complexity of this disorder. Despite this complexity, many of the metabolic alterations that are seen in obesity are consistent with altered activity of the sympathetic and parasympathic nervous systems. This proposal focuses on one aspect of this altered activity -- the serotonergic input to the ventromedial nucleus (VMN) of the hypothalamus which has been shown to be reduced in obese Zucker rats. The two major objectives of this proposal are related to the general theme that altered VMN serotonergic activity is an important contributor to the altered regulation of energy balance that occurs in obesity. The first objective of this study is to determine if the altered VMN serotonergic activity of obese rats occurs before increased adiposity, decreased thermogenesis, or hyperinsulinemia. For this, we will examine 2, 4, 7 and 12 day old Zucker/Brown Norway (ZBN) obese (fafa) vs. lean (Fafa) pups. The use of ZBN pups, rather than Zucker pups, allows one to distinguish fafa from Fafa littermates at any age because of the presence of a restriction fragment length polymorphism closely linked to the fa locus. This is a major advantage of the ZBN hybrids because it has not been possible to reliably distinguish fafa from Fafa pups in the Zucker strain before day 7 of age. The ontogeny data from the ZBN pups will indicate if the altered VMN serotonergic activity of fafa rats occurs early in the sequence of events initiated by the mutant fa gene. The second objective of this study is to evaluate three possible mechanisms for the attenuated VMN serotonergic activity in fafa rats: (1) altered reception and/or transduction of signals arriving at the raphe (site of origin of serotonergic projections to the VMN); (2) altered release and/or reuptake of serotonin in the VMN; and (3) altered input to the dorsal raphe. These mechanisms will be evaluated using in vivo (microdialysis) and in vitro (slice) preparations from 12 wk old Zucker fafa vs. FaFa (lean) rats. This study will enhance our understanding of the mechanisms underlying the altered autonomic signaling associated with the abnormal nutrient partitioning characteristic of obesity.